Metallic Nanowire Arrays and Methods for Making and Using Same

ABSTRACT

Freestanding metallic nanowires attached to a metallic substrate are disclosed. A method of creating the nanowire structure using an anodized layer is presented. In one embodiment an optical SERS sensor is formed. The sensor head has at least one array of nanowires chemically functionalized to recognize molecules of interest. A method of forming a SERS sensor and using the sensor to analyze a sample is presented.

This non-provisional patent application claims priority fromnon-provisional patent application Ser. No. 11/206,632, filed on Aug.18, 2005, entitled Metallic Nanowire Arrays and Methods for Making andUsing Same which claims priority from provisional application Ser. No.60/603,203, filed Aug. 20, 2004, entitled Nanowire Optical Sensor SystemAnd Methods Of Use Thereof, both of which are incorporated herein byreference in their entirety.

This invention was supported in part by U.S. Government contract numberPhase I SBIR Navy Contract N65540-03-0055 and NSF Phase I DMI-0339668and portions of this invention are subject to a paid-up license to theU.S. Government.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention generally relates to arrays of nanowires attachedto a substrate that thereby form an array. Throughout this disclosurethe term “nanowires” refers to high aspect ratio, solid wire structures(made from, for example, a metal or a semiconductor) having a length inthe range from about 5 nm to about 5 microns with appropriate,application specific diameters. It is preferred that nanowires are madeof a metal, such as for example, silver, nickel, iron, gold, cadmium orcopper. In an additional embodiment, semiconductors for use as nanowiresare silicon, Germanium and GaAs. Nanowires in an array can all be of thesame length, or vary in length. Arrays of nanowires are a usefulstructure. In one embodiment, the nanowire array structure is used as anSERS sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a planar array of free-standing nanowires attachedto the surface of a substrate.

FIG. 2 Schematic showing the template process steps for growingfree-standing nanowires on a substrate.

FIG. 3 Schematic diagram of SERS application of nanowire arrays, wherethe nanowire array is grown on an optical fiber.

DETAILED DESCRIPTION OF THE INVENTION

The self-organizational properties of certain materials (e.g., anodicaluminum oxide (AAO)) are useful for nanowire production in the presentinvention. When aluminum is anodically oxidized in an acidicelectrolyte, a uniform and oriented porous-structured layer, i.e.,nanobores, is formed with nearly parallel pores organized in a hexagonalgeometry. Thus, in an aspect of the present invention, nanowires may beformed on a substrate in part by coating a surface with a desiredthickness of aluminum (for example, 1-3 microns) and then anodizing thealuminum metal forming a porous AAO template. Aluminum may be sputtercoated onto surface of substrates. Prior to aluminum coating, a thin(e.g., about 10-100 nm) strike layer of a desired metal, such as gold,may be deposited onto surface of an optical fiber substrate for lateruse as the electrode for electrochemically forming nanowires, i.e., forthe electrodeposition of gold to form gold nanowires. In this way,freestanding metal nanowires can be attached to a metallic substratelayer where the metals are substantially the same. Optically transparentconductors may also be used as strike layer electrode, e.g.,indium-tin-oxide (ITO). Other such optically transparent materials thatare useful in the present invention are ZnO:M and V₂O₅.

The versatility in processing allows nanowire arrays to be formed withuniform wire density using a variety of substrates, including flat solidsubstrates or, the outer surface of an optical fiber. Substrates mayinclude any of the optical materials well known in the art for use inoptical systems, such as glass, sapphire, etc., or other conductivesubstrates. Substrates may also include conductors and metals. Inaddition, one can control the length and diameter of nanowires in anarray, which facilitates tuning the array and optical sensor applicationto a particular excitation wavelength. Additionally, the density ofnanowires may be chosen to provide an array with a number of nanowiresper unit area adapted to maximize an output signal. For example, thelengths may range from 5 nm to 200 microns, preferably up to 100 micronslong, and wire densities can be up to 10¹¹ cm⁻² or even up to 10¹² cm⁻².

The AAO can be produced with the desired nanobore diameter, spacing anddepth to control the dimensions of nanowires formed on AAO template. Thedepth of nanobores is controlled by the duration of the anodizationprocess. The separation of nanobores is controlled by the anodizationvoltage and nanobore diameter is controlled by the duration of a postanodization chemical etch process, as will be disclosed in furtherdetail herein below. The array of nanobores is formed by self-assembly.For example, arrays of gold (Au) nanowires may be produced using porousAAO with nanobore diameters of about 20-80 nm, nanobore depths of about1-5 μm, and a center-to-center spacing of about 20-250 nm. Spectralanalysis performed on the nanowire arrays can be used to determine theproper dimensions that will optimize these parameters for plasmonresonance.

A SEM micrograph of a porous anodic aluminum oxide template will revealthe relationship between nanobore diameter and anodization voltage. FIG.2 shows a cross-section of AAO template as produced on aluminum metal.As the aluminum layer is anodized, AAO physically forms in a uniformporous formation to create template (FIG. 2)(5). The structure can beused directly to template nanowires. A barrier layer can be removed toform nanowires in contact with aluminum metal, or the oxidation can becarried out until all the Aluminum metal is consumed and nanoborescompletely penetrate the AAO structure.

In a representative anodization process, the aluminum layer, FIG. 2A, isanodized in a solution of 0.3-wt % oxalic acid at 2° C. The anodizationis carried out until all the aluminum metal is consumed and nanobores inalumina template penetrate through to strike layer, FIG. 2B. The typicalanodization rate is about 1 microns/hour. Anodization is performed atabout 20-1000 V DC depending upon the desired nanobore diameter andspacing. A brief post anodization etch in phosphoric acid will removeany residual AAO from the bottom of nanobore, exposing strike layer. Forexample, following anodization, the AAO nanobores can be widened andstrike layer cleared in a 0.5 wt % solution of phosphoric acid. It ispossible to accurately control the diameter and center-to-center spacingof nanobores by adjusting the anodization voltage, and the electrolytecomposition and concentration. For example, by optimizing voltage,electrolyte composition and concentration, and nanobore widening, thediameter and pitch may be controlled over a range of about 10-250 nm.Nanobore depth is often a linear function of the anodization time andcan be extended up to several hundred microns.

Metal nanowire arrays, which in one embodiment are made from Gold (Au),may also be formed using the basic AAO templating technique shownschematically in FIG. 2 except the anodization is carried out untilnanobores completely penetrate aluminum metal. Briefly, the first stepis to create porous AAO template structure, FIG. 2B. The next step fillsnanobores in template with metal by e.g., electrodeposition techniques,FIG. 2C. The anodized layer is partially or totally removed by chemicaletching using phosphoric acid, leaving an ordered array of nanowire tipsprotruding from an AAO matrix of nanobores, FIGS. 2D and 2E. Nanowiresmay be formed using Ag, Ni, Fe, Au, and Cu. In one embodiment thisproduces the SERS active surface. The AAO can be further removed whilestill maintaining ordered, well-aligned nanowires FIG. 2D (2). Furtherremoval of the AAO exposes ever-increasing lengths of nanowires. If theAAO structural support is completely removed, nanowires may collapse.However, if the nanowires bond to the substrate, they can remainfree-standing. FIG. 2E shows the free-standing nanowires (2) attached tothe substrate (3). The nanowire array structure is shown in FIG. 1, withthe free-standing nanowires (1) attached at one end to the substrate(2). In one embodiment, the length and diameter of nanowires may be suchthat it most effectively leads to Raman enhancement. Preferably, arraysof gold nanowires are produced about the circumference of optical fiber.Gold nanowires may be from about 20 nm to about 80 nm in diameter, witha center-to-center spacing of about 20-250 nm, and an exposed length ofabout 1-5 microns.

In an embodiment of the invention, gold nanowires may be prepared insideporous AAO templates by standard AC electrolysis conducted at, forexample, about 100-1000 Hz at a level of about 0.1-30 V_(ac) using afunction generator. Gold nanowires may also be prepared inside templateusing a DC electrodeposition technique, for example, using 1.2 Vdcapplied to the substrate as the cathode in the electrodeposition bath. Atypical electrodeposition bath may have a solution of potassium goldcyanide and citric acid that is pH controlled using potassium hydroxideand phosphoric acid. Alternatively, a gold sulfide plating solution canbe used. After plating, the AAO matrix can be partially etched back inphosphoric acid to expose a desired length of each nanowire, shown inFIG. 2D. In a preferred embodiment, nanowires will have direct opticalcontact with optical fiber, see FIG. 3 (4). Optical fibers that arecoated with nanowire arrays may be imaged using SEM at various stages ofproduction process to determine their dimensions and structuralintegrity and to document the work. By way of further example, Ag, Ni,Fe, Au, and Cu nanowire arrays may be patterned on a variety ofsubstrate materials using the AAO templating technique. Nanowire arraysusing porous AAO as template have been successfully engineered usingelectrodeposition of cadmium, iron, gold, silver, copper, nickel, andother metals from aqueous solution. Nanowire arrays can also be builtusing electronics fabrication methods such as photolithography andelectrodeposition.

In another specific aspect, the present invention provides an SERSoptical sensor with nanowires grown on a suitable substrate within aremovable template such as a removable self-assembled template. Forexample, an alumina template can be used in which a series of blindholes or nanobores have been formed, e.g., by etching (FIG. 2).Nanowires are grown within nanobores in alumina template formedaccording to a method of the invention using electrochemical processesso that nanowires are individually attached to substrate 20 aftercomplete removal of alumina template (FIG. 2). A conductive layer and aglass substrate base may be disposed below template (FIG. 2.)(4).

EXAMPLES

The following examples further illustrate the present invention, but ofcourse should not be construed as in any way limiting its scope. Theexamples below are carried out using standard techniques, that are wellknown and routine to those of skill in the art, except where otherwisedescribed in detail. The examples are illustrative, but do not limit theinvention.

Example 1 Engineering of Gold Nanowires on Glass Substrates

An AAO template and gold nanowires of 90 nm diameters, 2 μm long wereproduced as follows:

The base material used is a multilayer structure comprising a standard75 mm by 25 mm glass microscope slide, a layer of indium tin oxide (10to 20 Ohms per square), and a layer of aluminum 1 to 1.5 microns thick.The gold nanowire arrays were produced on these substrates using thefollowing procedure:

1. Prepare anodization bath using 0.1 to 0.3 weight percent oxalic acidsolution. The concentration of the solution used to anodize the aluminumlayer is dependent on voltage being used to anodize. Higher voltagesrequire lower concentrations. Cool the bath to 5° C. by placing in anice bath.

2. Clean aluminum substrate with acetone and alcohol followed by a DIwater rinse for 1 minute.

3. Blow dry with air.

4. Mask off interface line on the sample with acrylic paint to protectareas not to be anodized.

5. Set up the data acquisition system to acquire data during processing.

6. Set power supplies to the required voltage.

7. Place the sample in the anodization bath so that the masked areafalls slightly below the liquid-air interface of the bath.

8. Connect electrical connections. Positive lead attached to aluminumlayer to be anodized, negative lead to a stainless steel mesh counterelectrode.

9. As the initial oxide layer forms on the surface of the aluminum themeasured current level drops rapidly to a “resting” level whereanodization takes place.

10. As the anodization front reaches the back of the aluminum layer thecurrent level will decrease slightly, and then start to increase. Theincrease in the current is associated with the thinning of the barrierlayer at the bottom of the channels as it is consumed by the anodizationprocess.

11. When the current level reaches twice the “resting” current, thepower is removed from the cell to stop the anodization process. Thesample appears uniformly translucent at the end of the process.

12. The sample is removed from the anodization bath and rinsed with DIwater for 1 minute.

13. The sample is placed in a beaker filled with DI water to keep theanodized channels filled with water. This is important to achieveuniform widening of the channels as well as uniform filling during theplating process.

After the porous matrix has been formed in the aluminum layer thechannels are widened using a pore widening etch. The etch serves twopurposes: first, to widen the channels to the desired diameter andsecond, to remove the remainder of the barrier at the bottom of thechannels to provide direct electrical contact to the ITO layer forelectrodeposition. A specific process used to widen the pores createdfrom the above anodization procedure is:

1. Prepare a 5 vol % phosphoric solution for pore widening.

2. Heat the solution to 37° C.

3. Place sample in the phosphoric solution so that the edge of themasked area lies below the liquid-air interface.

4. Allow the sample to remain in the bath for 7-10 minutes using mildagitation.

5. Remove sample and rinse in DI water for 1 minute.

6. Return sample to DI water bath to keep pores wet before plating step.

Electrodeposition of metals into the channels is accomplished using astandard electroplating bath. All metals, semiconductors and insulatorsthat can be deposited using an electrodeposition process may bedeposited into the channels. The specific materials of interest are thetransition metals most notably copper, silver and gold. Below weenumerate a specific process used to deposit gold into the porousmatrix. These can also be useful for one embodiment of the invention inthe form of an SERS sensor

1. Pre-Heat TSG-250 gold sulfite plating solution to 55-60° C.

2. Turn on the power supply and set up plating parameters:

a. Frequency 20 kHz; b. Amplitude 400 mV; c. Offset 800 mV

3. Place the sample in the plating bath so that a portion of the maskedarea lies beneath the liquid interface.

4. Connect electrical leads. Positive to a gold anode and negative tothe ITO conductive layer on the sample.

5. Plate the sample for 2 minutes 30 seconds with mild agitation.

6. Remove sample from plating bath.

7. Rinse residual plating solution from the sample for 1 minute followedby air blow dry.

Once the channels have been filled with metal, the remaining aluminamatrix is removed using the following process:

1. Heat a 5 vol % phosphoric solution to 37° C.

2. Place the plated sample in the solution so that the masked area isinserted just beneath the solution-air interface.

3. Etch the sample for 45 to 60 minutes.

4. Remove the sample from the phosphoric solution and rinse in DI waterfor 1 minute.

5. Allow to air dry to prevent damage to the wire array.

Example 2 SERS Application

One embodiment of the present invention discloses a surface enhancedRaman scattering analysis system and methods for detection of targetmolecules in a test sample using the analysis system and a sensor forthe same. The system has a SERS sensor containing an optical substrateand an array of often functionalized high aspect ratio nanowiresdisposed on the optical substrate. The nanowires are preferablycylindrical and/or solid structures, and are often formed from a metal,such as, silver, nickel, iron, gold, cadmium, copper, or the like, or asemiconductor, such as silicon, Germanium, GaAs or the like. Typically,the nanowires have a length ranging from about 5 nm to about 5 microns.An illumination source (e.g., diode laser excitation source or opticalfiber laser excitation source) and optical data collection portion mayalso be included as part of an analysis system. The nanowires are oftenchemically functionalized so as to detect molecular species orbiological agents in a test sample.

Raman spectroscopy (relying upon Raman effect) provides definitiveinformation about the molecular structure of a material by investigatingits vibrational spectrum. Different molecular species exhibit differentRaman spectra. In fact, isomers of the same molecular species can bedistinguished by this technique.

A method for detection of molecular species or biological agents in atest sample is also provided that uses the analysis system of thepresent invention having a SERS sensor formed in accordance with oneaspect of the invention. The method includes contacting the nanowires onthe sensor with a sample to be analyzed, illuminating an opticalsubstrate, and collecting optical data from the system following theillumination.

Nanowires in a SERS optical sensor often comprise dimensions thatcoincide with a fundamental resonance of the exciting optical wave or aharmonic (integer multiple) of the fundamental resonance mode and/or cancreate a resonant cavity for the exciting optical wave or for surfaceplasmons in the nanowire. SERS optical sensor often includes nanowireswith a geometry adapted for plasmon field enhancement and largereduction of plasmon damping. A typical SERS optical sensor formed inaccordance with one embodiment of the present invention comprises anarray of nanowires formed about a surface of an optical fiber serving asa substrate (FIG. 3). Arrays of nanowires are often produced using atemplating technique (FIG. 2) that is dependable and relatively easy toincorporate into a manufacturing environment.

Surface Enhanced Raman Scattering (SERS) techniques may employ smallstructured materials. Noble metal nanoparticles exhibit very strongoptical absorption in the ultra-violet through the visible range of thespectrum, which is not observed in their bulk counterparts. Theabsorption leads to tremendous electric field enhancement at theparticle surface and in the regions between neighboring nanostructures.This field enhancement affect is utilized in SERS, by employingnanostructured materials to boost the Raman signal intensity. SERStechniques have often led to increases in the effective Ramancross-section by factors of 10¹⁴-10¹⁵, allowing the Raman spectra ofsingle molecules to be probed in relatively short times (e.g., tens ofseconds).

Referring to FIG. 3, a SERS optical sensor system is provided thatincludes an optical sensor comprising an array of nanowires fordetecting, discriminating, and quantifying molecular species usingspectroscopy methods. SERS optical sensor system often uses anillumination or excitation source, SERS optical sensor and an opticaldata collection and analysis portion. A diode laser or optic fiber lasermay often be used as excitation source. SERS optical sensor may beintegrated into a system including an optical detector. The preferreddetector for a SERS optical sensor system is photodiode array fabricatedfrom InGaAs technology. These InGaAs diode arrays will operate in thenear infrared allowing the use of high power diode lasers. The advantageof near infrared Raman is that luminescence that interferes with theRaman spectrum can be avoided since the near IR photon energy is too lowto generate these excitations. A portable version of SERS optical sensorsystem with handheld data collection and/or analysis units can be usedfor testing various samples.

Nanowires manufactured according to the methods of the present inventionare preferably chemically functionalized to recognize or to trapselective chemical species in a test sample. Functionalization involvesdepositing one or more suitable active chemicals on the surfaces of thenanowires. The chemicals may include, but are not limited tothiocyanate, thiol alkanea, peptides, proteins, antibodies (monoclonaland polyclonal), DNA, RNA, PNA, histidine, streptavidin, biotin andinorganic elements, ions or compounds and other suitable chemicalscapable of reacting or binding with counterparts in the test sample. Inone embodiment, nanowires are coated on their respective outer surfaceby submersing the array in a solution of the functionalizing material.One or more monolayers may be added to surface of each nanowire in thisway. Additional layers may include antibodies and antigens. In oneembodiment of the invention, SERS optical sensor has an array ofnanowires oriented normal, i.e., at or about 90°, with respect to thelongitudinal axis of an optical fiber (FIG. 3). The array of nanowirescan be used as a test platform for single or multiple chemical orbiological species, for drug testing, or for environmental testing.

The nanowire arrays may be functionalized in any number of solutions toform molecular bridges to other target molecules. Here we detail aprocess for functionalizing the surface of gold nanowires with potassiumthiocyanate (KSCN) as a bridging molecule for hemoglobin. In solutionKSCN dissociates to become K⁺ and SCN⁻.

1. An array of gold nanowires was immersed in a 1 molar aqueous solutionof potassium thiocyanate.

2. The nanowire array is allowed to sit in solution for 30 minutes toallow the KSCN molecules to fill all the locations on the surface of thegold nanowires. The sulfur atom in the SCN⁻ radical preferentially bindsto the gold nanowire.

3. The nanowire array is removed from the KSCN solution and rinsed in DIwater for 5 minutes.

Infrared data of the nanowire array after this treatment show that theSCN radicals are still present on the surface after rinsing, stronglysuggesting the presence of the SCN attached to the gold nanowire.

In part, functional groups disposed on nanowires depend on the sample tobe screened or the type of assays. Screening can involve detection ofbiochemical substances, such as proteins, metabolites, nucleic acids orbiological agents such as fungi, bacteria and viruses. For example,nanowires can be functionalized for applications in genomics, proteomicsand SNP assays, medical diagnostics, drug discovery screening, anddetection of biological and chemical warfare agents.

Specifically, a nanowire may be functionalized by attaching a bridgingmolecule and/or a reactive molecule to nanowire using solutionchemistry. The nanowire array is submersed in a solution containing aconcentration of the bridge molecules large enough to coat surfaces ofnanowires with a monolayer of the bridge molecule. In the specific caseof gold nanowires, the bridging molecule has a thiol (sulfur containing)group that preferentially binds to the gold surface. The opposite end ofthe bridging molecule contains a chemical group that preferentiallyattaches to additional bridge molecules or the sensor target.

A specific embodiment uses mercaptoundecanoic acid which contains both athiol group and a carboxylic acid group. The thiol group binds to goldnanowire and the carboxylic acid group can bind to antibodies orantigens specific to the molecule the sensor is targeting. Othermolecules can also be used. For example, potassium thiocyanate can beused as a bridge for hemoglobin detection. The functionalization processis simply finding the molecules that will create the chemical bridge andbind to both nanowire and the target/additional bridge molecules. Aspecific advantage of nanowire geometry is that the bridging moleculeswill populate entire surface of nanowire providing many more sites fortarget binding to occur. Binding along the entire length of nanowireenables a greater volume of target molecules to be sampled, increasingdetection efficiency.

The first step in functionalizing nanowire arrays may be the formationof a self-assembled monolayer (SAM). As an exemplary embodiment of theinvention, gold nanowire arrays may be chemically functionalized to bindwith hemoglobin. To functionalize the gold nanowire arrays topreferentially bind hemoglobin, a SAM is deposited onto surface of eachgold nanowire by reacting a sulfur group, for example, in thiocyanate orthiol alkane with the gold. The sulfur group of the thiocyanate ion(SCN⁻) will bind to surface of each gold nanowire. The iron in thehemo-group of the hemoglobin will bind to the cyanate portion of theion. For example, a 0.1 M aqueous solution of thiocyanate with a pH of 7may be used at room temperature.

If needed, a different SAM using a thiol alkane (carbon chain with asulfur group attached to the end) with a cyano-functional group on theterminal end (HS—CH₂—CH₂—CH₂— . . . CH₂—CH₂—CN) may also be deposited inplace of, or in addition to using thiocyanate. The cyano group will befacing away from surface of nanowires so that the length of the alkanechain can be controlled to accurately penetrate the protein. Differentlengths of the alkane chain functional groups may be prepared todetermine the optimum length that will reach into the hemo crevice tobind to the iron. For example, gold nanowire arrays may be submerged inan ethanol solution of 1 mM thiol alkane (or aqueous solution ofthiocyanate) for several minutes at room temperature. Nanowires are thenremoved from solution, rinsed with ethanol and dried under a stream ofnitrogen. This should achieve 90 to 95% coverage of nanowires with theSAM and the attached functional groups.

Characterization of the resulting thiol alkane (or thiocyanate) layercan be done using, for example, infrared reflection-absorptionspectroscopy and cyclic voltammetry. The infrared spectrum should showabsorption peaks due to the thiol alkane (or thiocyanate) that are notpresent on untreated gold nanowires. Cyclic voltammetry in a buffersolution employing modified gold nanowires as the working electrodeoften display oxidation and reduction peaks due to the presence of thiolalkane (or thiocyanate) on gold nanowires. These measurements areperformed on nanowire arrays with and without the functional groupsattached. After preparation of the SAM covered, functionalized goldnanowires, which will serve as the platform for the attachment ofhemoglobin, they may be stored under purified nitrogen.

After the functionalization has been performed, the ability of thecyanate group to bind to hemoglobin may be determined. For example, thiscan be accomplished by exposing (e.g., by immersing) a functionalizednanowire array to an aqueous solution of a commercial sample ofhemoglobin dissolved in a phosphate buffer of pH 7.4 and then measuringthe infrared spectrum at several positions on the surface of the array.Variations in the intensity of the infrared absorption from one positionto another is an indication of how uniformly the hemoglobin has bound tothe surface of nanowires. The presence of hemoglobin attached to thenanowires can be further verified using cyclic voltammetry to estimatethe percent coverage of the hemoglobin on surfaces of the nanowires.Imaging of the array with the attached hemoglobin may be also be doneusing SEM.

In one embodiment, SERS optical sensor has an array of nanowires on aflat solid substrate. Incident light from excitation source interactswith, and excites nanowire which in turn causes molecule (located on thenanowire surface due to that surface's prior functionalization) tovibrate and thereby give off Raman shifted emissions that are collectedby optical elements for further processing.

A large array of nanowires may be used to probe a sample of solutiondeposited onto the array. The nanowire array can act as a fluid wickspreading the solution throughout the array for maximum exposure of thesolution to surfaces. Different areas of the array may be functionalizedwith different surface treatments to attach one or more specificmolecules. In this way, the nanowire array may be used to analyze theconcentration of one or more species in a solution. Such an opticalsensor is often useful for drug testing where only small quantities ofdrugs are available for test.

In another embodiment, as shown schematically, an SERS optical sensorhas an array of nanowires on a flat transparent substrate, e.g., glass.Excitation light is directed through substrate to excite nanowires, withemissions from molecules on the nanowire side of substrate beingcollected by optical elements for further processing. Such a sensor maybe well suited for air and water monitoring systems for buildings orlarge public areas. Numerous plates of nanowires may be assembled into aparallel array with a single excitation source being used from outsidearray of plates. Since the plates are transparent to excitation source,the single excitation source can be used to probe many plates within anair or water stream without encumbering the stream. In the case of airsampling, a fluid, such as water, may be used to permeate the nanowirearray to improve collection of airborne chemicals and/or biologicalagents.

In yet another embodiment, an SERS sensor has an array of nanowires on aflat transparent substrate. Excitation light is guided by transparentsubstrate and escapes from the light guide and excites nanowires whichin turn causes molecule (located on the nanowire surface due to thatsurface's prior functionalization) to vibrate and thereby give off Ramanshifted emissions that are then collected by external optical elements.This embodiment may be well suited for a portable system whereexcitation source power is limited and must be preserved. Thisarrangement may also be well suited for reducing the effects of sensorfouling.

In a further embodiment, a SERS sensor has an array of nanowires on aflat transparent substrate. Excitation light is guided by transparentsubstrate, and escapes from this light guide to excite nanowires whichin turn causes molecule (located on nanowire surface due to thatsurface's prior functionalization) to vibrate and thereby give off Ramanshifted emissions. Raman shifted light emissions from nanowires,re-entering the light guide, are then collected. This arrangement may bewell suited for portable systems operating in a dirty environment wherefouling may inhibit both the excitation source and the generated Ramansignal.

In another embodiment, an SERS sensor has an array of nanowires on ashaped transparent substrate, preferably an optical fiber. Excitationlight escaping from this light guide excite nanowire which in turncauses molecule to vibrate and thereby give off Raman shifted emissionswhich are collected by external optical elements. This arrangement formsa low cost disposable sensor that may be bundled to form achemical/biological sensor suite in portable systems. A sensor maycomprise one or more such devices bundled together. Custom sensor headscan be constructed to provide custom sensitivity for one or more agentsof interest. A reflective enclosure can be used to collect all of thelight emitted from the different species, i.e., chemical sensitivities,in fibers in the array.

In yet another embodiment, a SERS optical sensor has an array ofnanowires on a shaped transparent substrate, preferably an opticalfiber. Excitation light is guided by the transparent substrate andexcitation light escaping from the light guide excites nanowires.Emissions from molecules, re-entering the light guide, are collected.This design is least affected by fouling where opaque and nonactivespecies settle on the nanowire array. This is the preferred embodimentwhen multiple excitation sources are employed to insure ultra highreliability for detecting a particular species in a diverse background.Multiple fiber sensors are bundled, each with an excitation source ofunique wavelength.

In another embodiment, a SERS sensor has an array of nanowires 6 on ashaped transparent substrate, preferably a hollow tube. Nanowires are onan inner surface with an outer surface of tube covered by a reflectinglayer. Excitation light is guided by the walls of tube. Excitation lightescaping from the light guide excites nanowires, and emissions frommolecules are collected by external optical elements positioned at oneend of tube.

In a further embodiment, a SERS sensor has an array of nanowires on ashaped transparent substrate, preferably a hollow tube. Nanowires are oninner surface with outer surface of tube covered by reflecting layer.Excitation light is guided by the walls of tube and excitation lightescaping from the light guide excites nanowires. Raman shifted lightemissions from molecules re-entering the light guide are collected.

In another embodiment, an SERS sensor has an array of nanowires on ashaped transparent substrate, preferably a hollow tube. Nanowires are oninner surface with outer surface of tube covered by reflecting layer.Excitation light may be introduced into the center of tube so as tointeract with, and excite nanowire which in turn causes molecule(located on surface due to that surface's prior functionalization) tovibrate and thereby give off Raman shifted emissions that enter thelight guide to be collected. This design allows for the maximumexcitation power level to be used and has the most efficient lightcollection. Primary use of this system may be for detecting traceamounts of agents in relatively clean gas/fluids.

Example 3 Raman Measurements to Determine the Sensitivity and Accuracyof Hemoglobin Detection

Raman spectra are acquired using the 514.5 nm line of an argon ionlaser. The light is directed against the surface of the wire array inthe backscattering geometry, but may be directed through the back sideof the glass substrate. Data are acquired for two to ten minutes by asingle grating monochrometer fitted with a linear CCD array with aresolution of 1 cm⁻¹. Referring again to FIG. 3, a Surface-EnhancedRaman Scattering (SERS) analysis system formed according to an aspect ofthe present invention operates as follows. A low-power diode laser thatis integrated into a fiber laser cavity creates a narrow bandwidthoptical excitation to probe molecular species located adjacent to SERSoptical sensor. Nanowires act as a type of optical antenna, transferringthe light energy from optical fiber 4 to molecules 8 located or trappedon surface of one or more nanowires in the array. This light thenexcites Raman emission from molecule. The Raman spectrum of themolecules provides a unique “fingerprint” for identification, as no twomolecular species have the same Raman spectrum. Nanowires formed to theappropriate geometry and dimensions enable the enhanced opticalsignature of molecular species 8 to be detected using SERS. Ramanshifted light that is collected back into optical fiber, while probelight is filtered out. For example, excitation source (e.g., diodelaser) laser is removed from the signal path using a notch filter orother similar optical device placed in the optical signal capture path.The separate, isolated optical path within optical fiber reducessensor-fouling errors. The emitted Raman spectra are then detected usingphotodiode spectrometer and the spectrum is analyzed in a manner similarto pattern recognition systems. Analyzer compares the detected spectrumto a database of known spectra, and quantifies the concentration ofdetected species. For example, such a system with nanowiresfunctionalized to bind hemoglobin can be used for detecting,discriminating, and quantifying the molecule in a drop of blood sampleused for analysis. A nanowire sensor head formed according to theinvention can be integrated with the other optical and electroniccomponents to form a compact and portable hemoglobin monitoring system.

SERS optical sensor system may be based upon arrays of metallicnanowires on a selected substrate (e.g., an optical fiber or othersubstrate material) and then chemically functionalizing nanowires tobind specific analytes. It will be understood that the present inventionmay be readily extended to detect and identify other importantbio-molecules, nucleic acids, or protein sequences. SERS optical sensorsystem offers dramatically increased speed and sensitivity for detectinga large range of biologically important molecules, as well as, thepotential for in vivo analysis.

Nanowire nanosensors such as optical sensor of the present invention candetect single molecules or chemicals at picomolar concentrations. One ofthe most practical physical properties of metallic nanowires for use insensor technology is plasmon resonance. Plasmons are collective electronoscillations that are triggered when electromagnetic radiation isincident upon a metal structure. The resonance is due to a correlationbetween the wavelength of the incident radiation and the dimensions ofthe metallic nanowires. The absorption of light often peaks at theplasmon resonance frequency. Without intending to be bound by anyparticular theory, it is believed that the enhancement is due to thewire geometry acting like a perturbation on a spherical nanoparticle toform a plasmon resonator with modes that lie predominantly along thelength of the wire. The nanowire geometry can be assumed to be similarto a prolate ellipsoid for the purposes of surface plasmon modes. In thepresent invention, nanowires have a size as to match the surfaceplasmaon modes to the laser frequency being used. Nanowires act asresonant cavities that enhance the strength of the electric field onsurface of each nanowire. The size of the enhancement is often directlyproportional to the quality factor for the mode. A cylindrical geometryof nanowire provides a distinct advantage to nanospheres in thatnanowires 6 can be polarized in a specific mode along their length.Nanowires polarized in this way can interact with neighboring nanowiresto increase the surface enhancement factor. Often there appear to be alinear relationship between the radius of a metallic cylindricalnanowires and the incident wavelength that governs the resonance effect.As classically derived, the resonant radius is a fraction of thewavelength, and for high-aspect cylinders, the resonance frequency isseemingly independent of the length. In these terms, high aspect rationanowires are usually height to diameter of about 5:1 or greater.

While some other factors, such as polarization, may be considered, thisexample demonstrates a key consideration in engineering nanowires todirectly couple light energy from optical fiber to target molecule.According to an aspect of the present invention, nanowires may be tuned(formed) to the proper diameter that corresponds to the excitationwavelength resonance used in the Raman analysis. Nanowires are oftengrown to a specific length, as defined by the depth of in template,i.e., the thickness of the anodized layer. The voltage used in thecreation of template controls the separation between the nanobores andhence the nanowires. The duration of a bore widening etch controls thediameter of nanobores and hence nanowire diameter. The particle sizedependence of the optical enhancement from nanoparticles in SERS hasbeen experimentally observed to have a linear relationship with theexcitation wavelength.

Nanowire geometry (high-aspect ratio, metallic, cylindrical structure)is desirable. For example, plasmon field enhancement may result due tothe large reduction of plasmon damping found in these structures.Further, SERS is more pronounced in composite groups of nanostructureslike arrays of nanowires. Importantly, organized particle arrays, unlikerandom structures, can be tuned to a common plasmon resonance frequencydue to coupling between the ordered structures in the array. Plasmonfield enhancement is typically large at the ends of nanowires and in theregions between nanowires due to strong electromagnetic coupling wherethe materials are touching or in close proximity.

Nanowires are often formed in the range from about 5 nm to about 200 nmin diameter and, preferably in the range from about 20 nm to 80 nm indiameter. Nanowires are often spaced between about 5 nm and about 50 μmwith a center-to-center spacing of about 20-250 nm. Nanowires formed onsubstrate may have an exposed length of about 1-5 microns to 10 microns.Nanowire densities may range from about 10⁹ to 10¹²/cm². With suchdensities and wire lengths of up to about 10 microns. Nanowire arraysubstrates can have a surface area enhancement factor of 10,000 forSERS, for example.

It is to be understood that the present invention is by no means limitedonly to the particular constructions herein disclosed and shown in thedrawings, but also comprises any modifications or equivalents within thescope of the claims. Although the present invention has been describedand illustrated in detail, it is to be clearly understood that the sameis by way of illustration and example only, and is not to be taken byway of limitation. It is appreciated that various features of theinvention which are, for clarity, described in the context of separateembodiments may also be provided in combination in a single embodiment.Conversely, various features of the invention which are, for brevity,described in the context of a single embodiment may also be providedseparately or in any suitable combination. It is appreciated that theparticular embodiment described in the Appendices is intended only toprovide an extremely detailed disclosure of the present invention and isnot intended to be limiting. The spirit and scope of the presentinvention are to be limited only by the terms of the appended claims.

1. A nanowire array comprised of a plurality of substantiallyfree-standing nanowires, each nanowire with a first end and second end;a substrate with a surface where the substantially free-standingnanowires are attached to the surface at the first end and thefree-standing nanowires are not supported by a layer of material that issubstantially distinct from the substrate material.
 2. The nanowirearray of claim 1 where the nanowires and the substrate are substantiallythe same material.
 3. The nanowire array of claim 1 where the nanowiresare made from one of silver, nickel, iron, gold, cadmium, silicon,Germanium or GaAs.
 4. The nanowire array of claim 1 where the nanowiresare made of copper.
 5. A nanowire array comprised of a plurality offree-standing nanowires each attached to a substrate, where saidnanowires are between about 1 micron and 10 microns in length and wherethe array exhibits nanowire densities between about 10⁹ and 10¹²nanowires per cm².
 6. The nanowire array of claim 5 where the nanowiresare made of the substantially the same material as the substrate.
 7. Thenanowire array of claim 5 where the nanowires are made of one of silver,nickel, iron, gold, cadmium, silicon, germanium or GaAs.
 8. The nanowirearray of claim 5 where the nanowires are made of copper.
 9. The nanowirearray of claim 2 where the nanowires are made of copper.